The present invention relates to a tandem type brake master cylinder for actuating the braking system of an automotive vehicle in which two independent braking systems are provided, and more particularly relates to such a tandem type brake master cylinder which is particularly well adapted to bleeding of the braking system to remove air therefrom.
There is a known type of tandem type brake master cylinder, which is constructed in outline as follows. A body is formed with an internal cavity defined by a large diameter cylinder bore and a small diameter cylinder bore coaxial with each other. A first piston member, formed with a large diameter land to engage with the large diameter cylinder bore and with a coaxial axially offset first small diameter land to engage with the small diameter cylinder bore, is fitted within this internal cavity of the body with the large diameter land slidingly mounted within the large diameter cylinder bore and the first small diameter land slidingly mounted within the small diameter cylinder bore. A second piston member, formed with a second small diameter land and a coaxial axially offset third small diameter land, is fitted within the internal cavity of the body with the second and third small diameter lands slidingly mounted within the small diameter cylinder bore. A first pressure chamber is thus defined between the large diameter land and the first small diameter land of the first piston member within the large and small diameter cylinder bores, a second pressure chamber is thus defined between the first small diameter land of the first piston member and the second small diameter land of the second piston member within the small diameter cylinder bore, and a third pressure chamber is thus defined between the third small diameter land of the second piston member and a closed end of the small diameter cylinder bore, within said small diameter cylinder bore. A first spring is fitted within the second pressure chamber and bears against the first piston member and the second piston member so as to bias the first and the second piston members apart so as to increase the size of the second pressure chamber, and a second spring is fitted within the third pressure chamber and bears against the second piston member so as to bias the second piston member in the direction away from the aforesaid closed end of the small diameter cylinder bore as to increase the size of the second pressure chamber. A first fluid outlet opens to the second pressure chamber, and this first fluid outlet leads to one of the dual independent braking systems of a vehicle to which this tandem type brake master cylinder is fitted, typically to the braking system for the rear wheels thereof. A second fluid outlet opens to the third pressure chamber, and this second fluid outlet leads to the other of the dual independent braking systems of the vehicle, typically to the braking system for the front wheels thereof. A means such as a one way valve or the like is provided for allowing fluid to pass from the first pressure chamber to the second pressure chamber but not in the reverse direction; this means may in fact be the seal between the first and the second pressure chambers, which typically is an annular rubber like seal mounted on the first small diameter land of the first piston member, which may be constructed so as to pass brake fluid from the first pressure chamber to the second pressure chamber. A means such as an appropriately positioned port is provided for communicating a brake fluid reservoir to the first pressure chamber when the first piston member is positioned at its extreme restored position; a means, which again may be an appropriately positioned port, is provided for communicating the brake fluid reservoir to the second pressure chamber when and only when the first piston member is positioned at its extreme restored position; and a means is provided for communicating the brake fluid reservoir to the third pressure chamber when and only when the second piston member is positioned at its restored position under the balancing action of the first and the second springs when the pressure in the second pressure chamber is substantially equal to the pressure in the third pressure chamber and also the first piston member is positioned at its extreme restored position.
As is well known, such a tandem type brake master cylinder can provide good and balanced braking action for both independent braking systems of the vehicle, when the first piston member is forced in the direction towards the second piston member.
However, a difficulty arises with such a prior art type of tandem type brake master cylinder, during bleeding or degassing of the conduit system associated with the braking system connected to the third pressure chamber.
In such degassing or bleeding of the second braking system of the vehicle, pressure is repeatedly applied to and released from the brake pedal of the vehicle to which said tandem type brake master cylinder is fitted by the foot of an operator in order to expel air from the second (exemplarily the front) braking system of the vehicle, with simultaneous opening of a brake bleed nipple for said second braking system. In this case, the second conduit system communicated to the third pressure chamber is not kept in the fluid tight state. On the other hand, the first conduit system connected to the second pressure chamber is continuously in the fluid tight state because the pressure generated in this conduit system can not be instantly released in a short period in which repeated pedaling action is in the phase of releasing the brake pedal. Therefore, during this pumping action for bleeding or degassing of the conduit system connected to the third pressure chamber, the pressure of the second spring and the residual brake fluid pressure within the third pressure chamber together are not sufficient substantially to move the second piston member towards the first piston member so as to recover the volume of the third pressure chamber. Thus, when the depression action and the releasing action of the brake pedal are repeated several times, the second piston member may be quickly driven to its extreme position abutting against the closed end of the small diameter cylinder bore, i.e. may be bottomed, not moving thereafter, and no bleeding of the second conduit system may be effected.